In vitro fertilization seeks to duplicate, to a large extent, the conditions and processes normally occurring within the female reproductive system that are necessary to oocyte development, fertilization and early embryonic development. In the clinic and laboratory, IVF involves several discrete procedures, such as collection of the oocytes from the ovary of the mother, preparation of the sperm, fertilization, and, once fertilized eggs are identified, a period of early embryonic development, and then transfer of the embryo to the mother's uterus. Each of these steps can take place over extended periods of time, during which the individual cells involved have a continuing need for nutrients, and are subjected to significant stress as a result of clinical manipulation and changed environmental conditions.
During IVF, a culture medium is ordinarily used as a substitute for the fluid secreted by the female reproductive tract that would ordinarily surround the gametes, zygote, and developing embryo. Most laboratories carrying out IVF use a single culture medium throughout the various procedures involved. In a number of laboratories, there has been a tendency to use tissue culture media for IVF procedures, such as Ham's F-10, which is formulated to support somatic cell growth, not gamete or embryonic cell growth. Tissue culture media generally are complicated systems, containing an array of amino acids, vitamins and other constituents. They can contain components that significantly impair embryonic development and viability after transfer. Further, to the extent tissue culture media contain components that are generally needed by the gametes and the embryo, the media are not formulated to provide the components at levels appropriate to support healthy gamete and embryonic development.
Other laboratories have used simple culture media, consisting of balanced salt solutions supplemented with carbohydrate energy sources such as glucose, pyruvate and lactate. Examples include Earle's, T-6, and human tubal fluid (HTF). These media generally lack certain key components found in the female reproductive tract, such as non-essential amino acids, and their constituents are not formulated in concentrations that meet the specific changing needs of the gametes and developing embryo at various stages of their development.
The two types of culture media commonly used for IVF generally are only capable of supporting embryonic development to the eight-cell stage. Such media are ordinarily not capable of supporting and producing a viable blastocyst stage embryo, complete with an epithelium and competent inner cell mass. Accordingly, embryo transfer, the return of the fertilized oocyte to the uterus of the mother, usually occurs at around the four-cell stage (day two) or around the eight-cell stage (day three). This is a time when the four- or eight-cell embryo would not typically have arrived in the uterus of the mother, if fertilization had occurred in vivo. Embryo transfer at this time involves placing the cleavage stage embryo in an environment oriented to a blastocyst stage embryo. The cleavage stage embryo must then undergo further development in a non-homologous environment to reach the blastocyst stage, in which the embryo has trophectoderm cells capable of implanting in the uterine lining.
Recent research and human trials have led to the development of two new culture media, G1 and G2, which represent significant advancements in adaptation of culture media to the physiological needs of the cleavage stage embryo and the embryo in the eight-cell through blastocyst stage of development. These media are described in the following publications: Barnes, Crombie, Gardner, et al, Blastocyst Development and Birth After In-vitro Maturation of Human Primary Oocytes, Intracytoplasmic Sperm Injection and Assisted Hatching, Human Reproduction, vol. 10, no. 12, pp. 3243-47 (December, 1995); Gardner and Lane, Culture and Selection of Viable Blastocysts: A Feasible Proposition for Human IVF?, Human Reproduction Update, Vol. 3, No. 4, pp. 367-82 (1997); Gardner, Vella, Lane, et al, Culture and Transfer of Human Blastocysts Increases Implantation Rates and Reduces the Need for Multiple Embryo Transfers, Fertility and Sterility, Vol. 69, No. 1, pp. 84-88 (January 1998). Use of these media, and particularly the G2 medium, supports the growth and development of viable blastocyst stage embryos in vitro. Accordingly, the development of these media paves the way for new approaches to embryo transfer to the uterus at the blastocyst stage, when the embryo is most adapted to surviving in the uterine environment and has developed structures and capabilities required for implantation to take place. Research utilizing the G1 and G2 media, and embryo transfer at the blastocyst stage, suggests that these media contribute to higher pregnancy rates, and reduces the need for transfer of multiple embryos and the risk of multiple births. Neither of these media, however, is optimized for supporting the gametes, oocyte maturation, or fertilization.